1999 Windstar-stalls at idle

Today my wife's Windstar wouldn't idle when she left work. Started fine but as soon as she released the gas pedal it would stall. I taped a matchbook cover to the idle stop at the end of the accelerator cable to get it home. During the ride home the idle returned to normal and it was fine the rest of the evening. I looked thru the shop manual but didn't see any info related to this problem. It doesn't look like the idle speed is adjustable? Any ideas or help would be appreciated.